1. Field of the Invention
The present invention relates to a technique for performing diopter correction by inserting or retracting a diopter correction lens.
2. Description of the Related Art
A fundus camera that observes and photographs a fundus of an examinee's eye needs to respond to a very wide range of diopters of the order of 40 to 60 diopter that corresponds from a severe myopia to a severe hyperopia of the examinee's eye. However, it is difficult for a focusing lens of the fundus camera to respond to a range as wide as 40 to 60 diopter only by a movement of the focusing lens, in terms of the physical space of an optical arrangement.
Thus, an auxiliary lens having a positive or negative power referred to as a diopter correction lens is inserted into an observation photographing optical system. Accordingly, if an amount of a mechanistic movement of the focusing lens is narrow, the diopter correction lens enables the fundus camera to cover each of a region of the severe myopia, a region of the severe hyperopia and a region therebetween (i.e., a region where the diopter correction lens is not inserted), so that the fundus camera can respond to the wider diopters of the examinee's eye. In a general fundus camera, when the fundus of the examinee's eye cannot be brought into focus while the diopter correction lens is absent, an examiner inserts the diopter correction lens as appropriate to perform focusing.
On the other hand, in a technique discussed in Japanese Patent Application Laid-Open No. 2007-37897, two configurations for eliminating the need for an examiner to insert and retract a diopter correction lens are discussed. In the first configuration, diopter data of an examinee is input into the fundus camera in advance, and the fundus camera determines whether the diopter correction lens is needed or not based on the diopter data. If it is determined that the diopter correction lens is needed, the diopter correction lens is inserted without the need for the examiner to operate. In the second configuration, when the focusing lens comes to a predetermined position, the fundus camera determines that the diopter correction lens is needed, and then the diopter correction lens is inserted.
Although the diopter correction lens is very useful because it can enlarge a diopter correction range in which focusing can be performed with a simple configuration, there is an issue as follows. More specifically, when the diopter correction lens is inserted from a state where the diopter correction lens is absent, the diopter of the observation photographing optical system of the fundus camera will be greatly changed, and operability thereof will be impaired by the fact that continuity is lost in the diopter correction of the fundus camera.
This issue will be described below with reference to FIG. 6. An upper part in FIG. 6 indicates a diopter correction range of the fundus camera, when the diopter correction lens is absent. It indicates that a diopter correctable range is −10D to +10D. A middle part indicates the diopter range when a diopter correction lens having a negative power is inserted. In this case, the diopter correctable range is changed to −25D to −5D by inserting the diopter correction lens having the negative power. A lower part indicates the diopter range when a diopter correction lens having a positive power is inserted. In this case, the diopter correctable range is changed to +5D to +25D by inserting the diopter correction lens having the positive power.
For example, when an examiner photographs an examinee with the severe myopia (−15D), the examiner tries to move the focusing lens to −D side to bring into focus because the examiner has no idea of a diopter of the examinee. However, if the examiner determines that it cannot get focus at a position of 8D, for example, the examiner inserts a negative diopter correction lens. Thus, as illustrated in FIG. 6, a diopter of the fundus camera is changed from −8D to −23D, and continuity of the diopter correction is lost. For this reason, the examiner would wonder which direction to move the focusing lens. The same thing can be happen in a case of eyes with severe hyperopia which need the positive diopter correction lens. For example, upon determining that it cannot be brought into a focus at a position of +8D, the examiner inserts the positive diopter correction lens. Then, as illustrated in FIG. 6, the diopter of the fundus camera is changed from +8D to +23D, and thus continuity of the diopter correction is lost.